C. Barus — Compressibility of Colloids. 295 



rupture (fig. 3), until finally (fig. 4) the elastic resistance breaks 

 down and axial rupture is the result. 



To account for the motion of the projectile in fig. 4, it is to 

 be remembered that the colloid is solid however soft it may be, 

 and that therefore, in fig. 1, pressure is not transmitted upward 

 appreciably more than a few centimeters above the meniscus. 

 The surfaces of like stress are conoids symmetrically aronnd 

 the axis and with their apices in it ; but these conoids rapidly 

 become more shallow and flatten out horizontally from the 

 meniscus upward. In fig. 4, however, the central parts of the 

 colloid between the projectile and the apex of the meniscus is 

 in a discontinuous or quasi-triturated state ; i. e. solidity has 

 here broken down with the advance of the projectile, tempora- 

 rily at least, even though the whole of this part of the colloidal 

 column is under pressure. The projectile has, as it were, 

 ploughed out a channel. 



Through this discontinuous or quasi-triturated canal pressure 

 is transmitted as through a fluid. Hence the projectile is 

 urged upward by hydrostatic pressure applied against its lower 

 hemisphere and transmitted through the channel in question. 

 The upper hemisphere of the projectile is pushed in this way 

 continually against the unbroken coagulated colloid, whose 

 elastic resistance is as yet too weak to resist the motion. 



But the discontinuous or triturated colloid behind the pro- 

 jectile gradually seals itself up under pressure, which is there- 

 fore transmitted less and less fully. The elastic resistances in 

 front thus gradually increase in relative effectiveness until rup- 

 ture is no longer possible. The strain ceases to break down, 

 the projectile stops. 



10. One of the chief characteristics of the phenomenon is its 

 repetition a dozen or more times, with gradually decreasing 

 intensity, even at approximately constant pressure. This is less 

 directly explained since the properties of the essentially com- 

 pressible coagulate with respect to viscosity and rigidity are 

 here brought into play, with the addition of a property to 

 re-cohere or re-cement under pressure. 



Pressure is brought up to the meniscus through mercury 

 transmission instantaneously. It is transmitted through the 

 coagulated colloid only to relatively short distances into the 

 column above the meniscus. When this is ruptured, pressure 

 is at once transmitted upward through the triturated channel, 

 but the intensity of pressure experienced at any axial point of 

 the colloid will be less as the point lies higher; for it is incon- 

 ceivable that pressure can be transmitted through the extremely 

 narrow channel of viscous body instantaneously. Thus a wave 

 or a single swell of pressure gradually moves upward. Now if 

 the colloid near the meniscus under full pressure has the 



